The present invention is related to a probe, more particular a probe tip configuration and a method of fabricating such probe tip configuration are disclosed. This probe can be used as a sensing element in a scanning proximity microscope and can also be used for characterization purposes in related fields.
Scanning proximity microscopes (AFM, STM, MFM) operate by scanning the surface of a sample with a probe. Such a probe can be used e.g. for topography measurements or as a nano-SRP (nano-Spreading Resistance) Probe, used for the determination of the resistance and carrier profile of a semiconductor element or for nano-potentiometry measurements of the electrical potential distribution on a semiconductor element. Such a probe usually consists essentially of a mounting block, a cantilever or stylus and a tip. The choice of the materials of which the mounting block, the cantilever and the tip are composed, strongly depends on the type of measurement the probe is intended for. For topography measurement a dielectric or a semi-conductive tip can be used whereas for resistance determination and nano-potentiometry a highly conductive tip preferably with high hardness and low wear is required.
In order to achieve a high resolution the tip of such a probe must have a very small radius of curvature. A classical probe tip, as e.g. described in the U.S. Pat. No. 5,399,232 consists of a single portion. Changing the dimensions of such a probe tip, in particular increasing the height, can be necessary to meet the specifications of the measurement setup in particular e.g. the tip to sample distance. An increase of the tip height leads to a significant increase of the aspect ratio. The mechanical stability of such high aspect ratio, tips is limited, which makes them less suitable for electrical measurements where these tips are exposed to high forces being applied in order to provide a good electrical contact. Therefore stable and reliable classical probe tips can only be fabricated for a limited range of dimensions. Furthermore there is an ongoing effort and interest in continuously improving the detection resolution and consequently the probe resolution, which will even further limit the availability of reliable probe tips.
In the prior art a probe tip configuration is suggested which partly overcomes this problem. The U.S. Pat. No. 5,455,419 describes a probe tip configuration where a tip is arranged on a pedestal. By dividing the probe tip into a pedestal and a tip, one can adapt the dimensions of the pedestal according to the specifications of the measurement set-up whereas the dimensions of the tip can be optimised independently. In U.S. Pat. No. 5,455,419 tip, pedestal and cantilever are composed of a single material. It is desirable to adjust the maximum penetration depth into the sample. Because of the special tip arrangement the penetration is limited to the tip while the pedestal can make contact with the sample surface without significant penetration. The maximum penetration depth can be altered by changing the dimensions, in particular the height of the tip. But because tip and pedestal are composed of a single material this easy adjustment of a maximum penetration depth into the sample is not applicable without drastically reducing the resolution. Furthermore the characteristics of both the pedestal and the tip can not be adapted to the physical quantity to be measured using different tip shapes and materials.
Also in the prior art a method of making such a probe tip configuration is suggested. U.S. Pat. No. 5,282,924 describes a method of fabricating said probe tip configuration where a tip is arranged on a pedestal. Said tip and said pedestal are composed of a single material which does not allow an easy adjustment of a maximum penetration depth into the sample. Furthermore the characteristics of both the pedestal and the tip can not be adapted to the physical quantity to be measured using different tip shapes and materials. A further drawback of this method is that there is no solution provided to realise a highly conductive probe tip. Tip, pedestal and cantilever beam are formed out of a single substrate material such as silicon. The cited document does not describe how to fabricate a probe with a highly conductive tip preferably with high hardness and low wear as desired for e.g. resistance determination. With cited method it is also not possible to fabricate a probe where probe tip and pedestal are isolated from each other e.g. for easy penetration depth adjustment or to reduce the capacitive coupling of the probe tip configuration to the sample by forming a pedestal which is composed of insulating materials.
In an aspect of the invention a probe, more in particular a probe tip configuration, is disclosed. This probe (FIGS. 1, 3) comprises a mounting block (3) and a probe tip configuration. Said probe tip configuration comprises a cantilever beam (1) and a tip, where said cantilever beam is fixed to the mounting block at one end and said tip is fixed to the other free end of said cantilever beam. Said tip comprises at least one small tip, hereafter designated as second portion of a tip (4), placed on a larger and preferably more truncated tip, hereafter designated as first portion of a tip (2), which is connected to said end of said cantilever beam. A probe tip configuration being part of a probe for use in a scanning proximity microscope is disclosed, comprising a cantilever beam and a tip, said tip being fixed to an end of said cantilever beam, said tip comprising a first portion of a tip and a second portion of a tip, said first portion of a tip being connected to said end of said cantilever beam, said second portion of a tip being placed on said first portion of a tip, characterised in that:
said cantilever beam, said first portion of a tip and said second portion of a tip form a structure comprising at least two different materials. This probe tip configuration can be used as a sensing element in a scanning proximity microscope. The special arrangement of the present invention allows the adjustment of a maximum penetration depth into the sample surface by changing the height of the second portion of a tip. The characteristics of the second portion of a tip can be adapted to the physical quantity to be measured using different tip shapes and materials.
In another aspect of the invention a probe tip configuration (FIGS. 2, 4) being part of a probe for use in a scanning proximity microscope is disclosed, comprising a cantilever beam (1) and a tip, said tip being fixed to an end of said cantilever beam, characterised in that,
said tip comprises a first portion of a tip (2) and more then one second portion of a tip (5), said first portion of a tip being connected to said end of said cantilever beam, said second portions of a tip being placed on said first portion of a tip. This probe tip configuration makes it possible to detect more than one signal of a sample at the same time using only one cantilever beam (1). The different second portions of a tip can be electrically and/or thermally isolated from each other and can have different characteristics. The different second portions of a tip can also be used to detect different physical quantities. In particular this probe tip configuration allows the use of one or more second portions of a tip as a current or voltage source. In this way, the other second portions of a tip can be used to detect the generated signal.
In a further aspect of the invention a method of fabricating a probe tip configuration is disclosed comprising a cantilever beam and a probe tip, said probe tip comprising a first portion of a tip and at least one second portion of a tip, said method comprising the steps of:
etching a substrate using a first patterned hard mask layer as an etch mask, said etching step creating a first etch pit;
etching at least one second small etch pit on the bottom of said first etch pit using a second patterned hard mask layer as an etch mask;
defining the cantilever beam by removing the exposed parts of said first hard mask layer;
covering the complete etched structure with at least one material thereby forming said first portion of a tip and said second portion of a tip;
etching through a mask window at the backside of said substrate to thereby form said cantilever beam and simultaneously etching free said first portion of a tip and said second portion of a tip.
In still a further aspect of the invention a method of fabricating a probe tip configuration is disclosed where the tip comprises a first portion of a tip and one second portion of a tip. The shape of the first portion of a tip is achieved by etching the substrate. The shape can be a truncated pyramid, a truncated cone, a cylinder. In particular a truncated pyramidal shape can be realised by employing an etching solution such as KOH for the etching of the substrate, which results in a more truncated pyramidal etch pit. Then a second portion of a tip can be formed on said first portion of a tip. Therefore a small etch pit on the bottom of the first etch pit is formed preferably by an anisotropic etching procedure resulting in a pyramidal or a conic shape. The complete etched structure is covered with one or more materials and the substrate is finally removed using a thinning window at the backside of the substrate resulting in a first portion of a tip and a preferably pyramidal or conic shaped second portion of a tip having a very small radius of curvature. First portion of a tip and second portion of a tip can also be composed of different materials.
In still another aspect of the invention a method of fabricating a probe tip configuration is disclosed where the tip comprises a first portion of a tip and at least two second portions of a tip. The shape of the first portion of a tip is achieved by etching the substrate. The shape can be a truncated pyramid, a truncated cone, a cylinder. In particular a truncated pyramidal shape can be realised e.g. by employing an etching solution such as KOH for the etching of the substrate which results in a more truncated pyramidal etch pit. Then at least one second portion of a tip can be formed on said first portion of a tip. Therefore at least one small etch pit on the bottom of the first etch pit is formed preferably by an anisotropic etching procedure resulting in pyramidal or conic shapes. The complete etched structure is covered with one or more materials and the substrate is finally removed using a thinning window at the backside of the substrate resulting in a first portion of a tip and a preferably pyramidal or conic shaped second portion of a tip having a very small radius of curvature in order to achieve high resolution. First portion of a tip and second portion of a tip can also be composed of different materials. The different second portions of a tip can be electrically and/or thermally isolated from each other.